This invention relates to nickel-base, single crystal superalloys for use in casting components for high stress, high temperature gas turbine applications.
This alloy and alloys of this type are intended for single crystal casting of gas turbine blades, particularly such blades used in maximum performance aircraft turbine engines where high resistance to thermal fatigue, creep and oxidation are essential.
The basic technology of alloys for the casting of single crystal components is described in U.S. Pat. Nos. 3,494,709, 4,116,723 and 4,209,348. An alloy materially improved over those described in these patents is described in my co-pending application for U.S. Patent Ser. No. 339,318, filed Jan. 15, 1982, entitled "Single Crystal (Single Grain) Alloy" U.S. Pat. No. 4,582,548. The alloys of said application are marketed under the trademark "CMSX" as CMSX-2 and CMSX-3 and are hereafter identified as CMSX-2 and CMSX-3. This invention is a creep strength/creep rate improvement on the alloys described in that application. It will be marketed under the "CMSX" trademark as CMSX-4.
Many factors enter into what constitutes an alloy having not only the desired mechanical and environmental performance features but also practical single crystal castability and heat treating characteristics. These two characteristics, i.e., component mechanical/environmental properties response and commercial producibility, normally conflict with one another in that alloy compositions which result in the desired performance features many times have impractical casting and heat treatment characteristics. Likewise, the reverse is also true.
Among the problems encountered is that of creating an alloy having sufficient creep-rupture and oxidation resistance while also exhibiting a heat treatment temperature range which permits it to be heat treated to a temperature at which all of the primary gamma prime goes into solution without the alloy reaching its incipient melting temperature. In addition, it is desirable that at least some of the eutectic also be solutioned. This range is called the heat treatment "window". Failure to provide a practical "window" such as 25.degree. F. or more makes it all but impossible to heat treat the castings without unacceptable diminishing mechanical properties or scrap rate due to either incomplete solutioning of the primary gamma prime or incipient melting. An alloy which cannot be effectively heat treated to total solutioning of the primary gamma prime without experiencing a significant rejection rate due to incipient melting is little more than a laboratory curiosity and is without practical utility. The alloys CMSX-2 and CMSX-3 described in U.S. patent application Ser. No. 339,318, U.S. Pat. No. 4,582,548, noted above, are alloys which overcome this problem. In a variation of the basic alloy (CMSX-2) described in that application, the coated oxidation/corrosion performance of the alloy was increased with the addition of hafnium (CMSX-3) while retaining a practical solution heat treatment window.
The present invention is the result of research having the objective of retaining the desirable commercial producibility characteristics typical of the CMSX-2 and CMSX-3 alloys while significantly improving the high temperature, high stress, creep resistance of the alloy.
Creep in the form of permanent, plastic deformation of an alloy is normally defined in terms of the percentage of elongation occurring at a given temperature/stress over a specific time period before rupture occurs. While increasing the time and temperature at which creep translates into rupture is important data concerning alloy characteristics, it is not particularly significant to the designer of high performance aircraft turbine engines. The designer is more concerned with the time and temperature range at which a specific percentage of creep will occur. This is important because it determines when and under what conditions a specific dimensional change in the casting will cause malfunction such as results from physical interference between engine components. As the performance characteristics of engine designs increase the clearance between components normally must be decreased, reducing the amount of creep which can be tolerated. At the same time, the increased performance characteristics normally necessitate higher temperatures and speeds, factors which increase stress and accelerate creep. In the modern, high performance, gas turbine engine, designers consider one percent creep the practical limit of acceptability.
The invention provides a nickel-base superalloy which retains the practical heat treatment "window", alloy stability and single crystal castability characteristics of the CMSX-2 and -3 alloys while very significantly improving creep resistance characteristics by increasing both the permissible operating temperature and the time before a one percent creep will occur. This has been done by increasing the refractory element components of the alloy, including the addition of rhenium. The invention achieves a very significant increase in creep resistance without creating alloy instability which can result from the incorporation of rhenium.